It's fairly clear it wouldn't have been the youngest brother. That leaves the elder brother. I mean, who else could do it then?
8 seconds sound clip from the Strife for Mastery (1978) movie soundboard.
You can hear this line at 01:09:25.902 in the DVD version of the movie.
Quote context
[...]
- Don't you worry. I shall keep my promise alright.
- Now we've carried out the first step. Proceed to the second. You know what.
- Sir.
- Who?
- Only Lord Wong's relatives could possibly have killed him.
- Did you follow my orders and kill all Lord Wong's men?
- Yes, sir, except for that son, all the others are dead now.
- Of course I will. I'll cure it by killing you.
[...]